Operating an engine with more than a single fuel allows an engine to operate in a way that may improve engine operation as compared to when only a single fuel type is available. For example, an engine operating with reformed fuel may be able to tolerate a higher level of cylinder dilution than an engine operating solely with gasoline. On the other hand, it may be desirable to operate the engine solely with gasoline when reformate is unavailable or in low supply. Thus, it may be desirable to adjust operation of an engine depending on an amount of available fuel. U.S. Patent Application 2008/0221778 describes a system wherein engine speed and load are set differently when a quantity of fuel stored in a second fuel tank is less than a predetermined value.
While it may make sense to adjust engine speed and load in response to an amount of fuel in a fuel tank, simply limiting engine speed and load may not leverage the remaining fuel in a way that improves engine operation with the remaining fuel. Further, although it may be desirable to limit engine speed and load, such limiting may provide little benefit if the engine is operating at high dilution levels. Further still, there may be times when it is desirable to over-ride such limiting functions.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method for improving gaseous fuel utilization. One embodiment of the present description includes a method for operating an engine, comprising: operating a fuel reformer and producing a gaseous fuel; and limiting a rate of injection of said gaseous fuel to at least an engine cylinder in response to an amount of gaseous fuel in a storage tank less than a threshold amount when said storage tank is not empty.
By limiting the rate gaseous is injected to an engine in response to an amount of gaseous fuel in a storage tank less than a threshold amount when the storage tank is not empty, it may be possible to extend the amount of time an engine may be operated at higher dilution levels. For example, if an amount of fuel stored in a storage tank is less than a predetermined amount, it is possible to reduce the level of cylinder charge dilution while at the same time maintaining a level of charge dilution that is greater than if the engine is operated without gaseous fuel injection. In addition, limiting injection of gaseous fuel may be overridden during some conditions even though the amount of stored gaseous fuel is low.
The present description may provide several advantages. Specifically, the approach may extend the range or time that an engine may operate at higher levels of cylinder charge dilution. Further, the method may allow a smaller fuel reformer to provide gaseous fuel to the engine, thereby reducing vehicle weight and cost. Further still, the method may improve vehicle emissions and fuel economy as compared to other systems that do not prioritize gaseous fuel utilization.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.